Method for processing a substrate surface and an apparatus therefor

ABSTRACT

According to the invention, a viscous substance is applied to the undulated surface of a substrate to be processed, and a gravity-like force greater than that of normal gravity is applied to the viscous substance in the direction orthogonal to the principal plane of the substrate. Consequently, the viscous substance applied on the convex parts flows into the concave parts under the influence of the gravity-like force, so as to fill up the concave parts with the viscous substance. As a result, the top of the surface of the substrate is substantially flattened. 
     To apply such gravity, the invention employs a rotor, and a substrate holder supported in part of the rotor in a rotatable manner, and by rotating the rotor at high speed, the substrate holder is automatically rotated, and the substrate mounted on the substrate holder is positioned in the direction orthogonal to the rotating plane of the rotor, so that a kind of gravity by centrifugal force may be applied to the viscous substance.

BACKGROUND OF THE INVENTION

The present invention relates to a processing method and a processingapparatus for treating a substrate surface such as that of asemiconductor substrate.

In manufacturing semiconductor devices it is often required to processthe substrate surface at high density and high precision, but when thesubstrate surface is extremely undulated as may result from a series ofsurface processing steps, it is often extremely difficult to process thesurface in subsequent steps. In such a case, a technique which issometimes employed is that of filling the recesses of the substratesurface with an appropriate material to make the substrate surface flatand smooth.

As an example, FIG. 5 shows a part of the surface of a solid state imagesensor. As shown in FIG. 5A, an impurity diffusion region 22, such as aphotodetector, is formed in a surface region of a semiconductorsubstrate 21, and then a transparent insulating film 23 is formed on thesurface of the semiconductor substrate 21. Although shown conceptuallyin FIG. 5, actually, silicon gates and oxide films must be successivelystacked up on the protective film 23 between adjacent diffusion regions22, 22. As a result, a concave part 24 is formed above the diffusionregion 22 of the semiconductor substrate 21, while a convex part 25 isformed between adjacent diffusion regions 22, 22.

It is difficult to form a color filter on an undulated surface, such asthat shown in FIG. 5A. Accordingly, as shown in FIG. 5B, the concavepart 24 is filled with an appropriate transparent material 26 to makethe substrate surface smooth and flat.

Later, as shown in FIG. 5C, a color filter 27 is formed on the flattenedsubstrate surface. The color filter 27 is composed by applying materials30 sequentially. These materials are selectively dyed with organicdye-stuffs on the basis of gelatine, casein or the like within theregion sectioned by interlayer 28 and separation layer 29. Subsequently,the surface is covered with protective film 31.

FIG. 6 shows another conventional substrate surface flattening method.

As shown in FIG. 6A, the surface of substrate 32 includes concave parts33 and convex parts 34. An insulating film (for example, an oxide film)35 is formed on the surface of the substrate 32.

On the surface of substrate 32, as shown in FIG. 6B, for example, atransparent polymer resin 36 having an approximate viscosity is applied.As a result, concave parts 33 are filled with polymer resin 36a. Polymerresin 36b also builds up on the surface of convex parts 34, however.

If polymer resin 36 is not photosensitive, as shown in FIG. 6C, aphotoresist 37 is applied on the surface of the polymer resin 36, andthe photoresist 37 is patterned by using a photomask (not shown). As aresult, the portion 37b corresponding to the convex portion 34 isremoved, and the portion 37a corresponding to the concave portion 33 isleft over.

When the substrate surface is then etched, as shown in FIG. 6D, thepolymer resin 36b on the convex part 34 is removed, and the concave part33 is filled with the polymer resin 36a and part 37a of photoresist.Afterwards, by removing the residual photoresist 37a, the substratesurface may be almost flattened. In this state, however, since thepolymer resin 36a builds up in the marginal area of the concave part 33,substrate surface is not perfectly flat.

Accordingly, the substrate 32 is heated to about 200° C. Then theviscosity of the polymer resin 36a is lowered, and its fluidity isincreased, and by the function of the gravity of the polymer resin 36amoves into the concave part 33. As a result, as shown in FIG. 6E, asmooth flat surface is formed on the substrate surface.

Meanwhile, when a photosensitive material is used for the polymer resin36, for example, a PMMA photoresist, the photosensitive polymer resin 36may be directly exposed and developed by using a photomask, withoutusing photoresist 37 as shown in FIG. 6C, so that the state in FIG. 6Bmay be directly formed into the state in FIG. 6E.

In such prior art, however, it is necessary to remove the material onthe convex part 34 by photolithography. In other words, it stillrequires a photomask to be matched with the undulation pattern on thesubstrate surface, and this photomask must be aligned with theundulation pattern of the substrate surface to be exposed and developed.Accordingly, due to the photolithography process steps, the number ofprocess steps required is increased and the yield may be reduced becauseof the risk of photomask defects or photomask shape defects induced bydust or the like.

It is hence a first object of the invention to provide a method forprocessing a substrate surface in order to form a smooth flat surface onthe surface of the substrate to be processed, by filling the concaveparts of the substrate surface with a substance, without resort tophotolithography.

It is a second object of the invention to provide a processing apparatusfor easily executing such substrate surface processing method.

SUMMARY OF THE INVENTION

In accordance with the present invention, a viscous substance is appliedto an undulated surface of a substantially planar substrate to beprocessed; a gravity-like force is then applied to the viscous substancein the direction orthogonal to the principal plane of the substrate.This causes the viscous substance applied on the convex parts of theundulated surface to flow into the concave parts thereof, therebyfilling up the concave parts with the viscous substance. As a result,the top of the substrate surface is substantially flattened.

To apply such gravity-like force in this process, the invention furtherincludes an apparatus comprising a horizontally disposed rotor and asubstrate holder rotatably supported in the marginal part of the rotor.By rotating the rotor at high speed, the substrate holder isautomatically rotated from its starting positing (in which a planarsubstrate held by the holder is substantially horizontally disposed)about the axis of its supporting shaft, which is perpendicular to theradius of the rotor, by the action of centrifugal force on a weight onsaid holder below the position at which a substrate would be heldthereon. A substrate held by the holder is thus positioned with itsprincipal plane orthogonal to the radius of the rotor, thereby applyingcentrifugal force to the viscous substance.

Other features and effects of the invention will be more clearlyunderstood and appreciated from the following detailed description ofthe preferred embodiments taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A 1B, 1C and 1D are process sequence sectional drawings showingthe processing method of a substrate surface accord to one of theembodiments of the invention;

FIGS. 2A and 2B are a side view and a plan view, respectively, showprocessing apparatus suitable for executing the method illustrated inFIG. 1;

FIGS. 3A and 3B are a plan view and a side view, respectively, of asubstrate holder used in the processing apparatus shown in FIG. 2;

FIGS. 4A and 4B are magnified perspective views of essential parts ofthe processing apparatus shown in FIG. 2;

FIGS. 5A 5B and 5C are sectional views showing essential parts of asolid state image sensor in the sequence of a conventional manufacturingprocess; and

FIGS. 6A 6B 6C 6D and 6E are process sequence sectional views alsoshowing a conventional processing method of a substrate surface.

DETAILED DESCRIPTION OF THE INVENTION

A method for processing a substrate surface in accordance with a firstembodiment of the invention is described in detail below while referringto FIGS. 1A to 1D.

As shown in FIG. 1A, the surface of a substrate 1 (such as asemiconductor substrate) to be processed includes concave parts 2 andconvex parts 3. Meanwhile, an insulating film 4 such as an oxide film isgrown on the surface of the substrate 1, but the insulating film 4 isvery thin as compared with the depth of the concave parts 2 or theheights of the convex parts 3. Accordingly, the concave parts 2 andconvex parts 3 remain on the surface of the substrate 1 as they are.

Next, as shown in FIG. 1B, a viscous substance 5 is deposited on thesurface of substrate 1. The viscous substance 5 may be applied, forexample, by spin coating of a photoresist as used in an ordinarysemiconductor manufacturing process, or by deposition of ordinaryphotoresist by an electrostatic coating or evaporation coating method.Of course, polymer resins other than photoresist may also be applied. Asin prior art processes, such as that described with reference to FIG. 5,the viscous substance 5 may be either transparent, translucent oropaque, depending on the purpose of the device. The requiredcharacteristics for the viscous substance 5 are adequate viscosity andproper fluidity, and other characteristics are not particularly defined.In FIG. 1B, the coating weight of the viscous substance 5 should bepreferably proper, neither excessive nor insufficient, in considerationof the following cautions. That is, as shown in FIG. 1B, parts 5a of theviscous substance 5 sink along the surface of the concave parts 2, whileother parts 5b of the viscous substance 5 build up along the surface ofthe convex parts 3, but the viscous substance 5b on the convex parts 3flow into the concave parts 2 by the force induced flow processingdescribed below. At this time, it is desired that the total amount,specifically the volume, of the viscous substance 5a initially appliedin the concave parts 2 and the viscous substance 5b flowing into theconcave parts 2 by processing in accordance with this invention benearly equal to the amount necessary to fill the concave parts 2. If thetotal amount of the viscous substance 5 is excessive, the concave parts2 are brimed over with the viscous substance 5. To the contrary, if thetotal amount of the viscous substance 5 is too little, the concave parts2 are not filled up completely. As a result, in either case, a smoothflat surface will not be obtained.

Therefore, the volume of viscous substance used is from slightly lessthan to slightly more than (and preferably about equal to) to the totalvolume of the concavities in the undulated surface of the substratesurface to be treated.

As shown in FIG. 1B, after depositing a proper amount of viscoussubstance 5 on the surface of the substrate 1, a gravity-like force isapplied to substrate 1 in a direction orthogonal to the principalsurface of substrate 1 in FIG. 1C. This force is greater than 1G (normalgravity) and is preferably from 10 to several thousand times that ofnormal gravity. Then, the viscous substance 5b on convex parts 3 ismoved by the gravity-like force to flow gradually into concave parts 2from above convex parts 3, and viscous substance 5b on convex parts 3decreases as shown in FIG. 1C, while viscous substance 3a in concaveparts 2 increases. As this process continues, viscous substance 5b onconvex parts 3 will soon entirely flow into concave parts 2, and concaveparts 2 are filled with the viscous substance 5. As a result, thesurface of the substrate 1 is finally substantially flattened as shownin FIG. 1D.

Thus, according to the first embodiment of the invention shown in FIGS.1A to 1D, the substrate surface may be flattened without resort to aphotolithography process. Therefore, it is not necessary to use aphotomask; and a series of photoetching steps, requiring alignment ofsubstrate surface undulation pattern with a photomask, followed byexposure and development, is not needed. Hence, the productivity of thesubstrate processing may be enhanced, and the lowering of yield due tophotomask defects or photomask shape defects induced by dust or the likemay be prevented.

In addition, after flattening the substrate surface as shown in FIG. 1Cby processing with gravity-like force, it may be useful to apply plasmaetching on the entire surface of the viscous substance 5 in order toremove the uppermost portion of viscous substance 5. This results inremoval of viscous substance 5b on the convex parts 3 as shown in FIG.1D. At this time, although part of the surface of the viscous substance5a in the concave parts 2 may be also removed, it does not matter atall, as a practical matter, because a substantially flat surface isobtained. Particularly at the stage shown in FIG. 1B, if the totalamount of the viscous substance 5 is excessive, the viscous substance 5bmay be left over on the convex parts 3 and an excessive viscoussubstance 5a tends to flow over the concave parts 2. In such a case, itis beneficial for obtaining a smooth flat surface to uniformly removethe viscous substance 5a in the concave parts 2 and viscous substance 5bon the convex parts 3 by applying plasma etching on the entire surfaceof the viscous substance 5.

FIGS. 2A and 2B relate to an embodiment of an apparatus for processing asubstrate surface. This apparatus is suited for applying a force of fromten times to thousands of times that of gravity to the viscous substance5. At four positions on the circumference of a horizontally disposedrotor 6, rotatable about a vertical axis 0, substrate holder housings 7are disposed. At both sides (or chordal ends) of the substrate holderhousing 7, a pair of grooves 8, 8 are formed for supporting thesubstrate holder which is described below.

FIGS. 3A and 3B show the detailed structure of a substrate holder 9.Substrate holder 9 comprises a support member 10 of square shape, asubstrate holding part 11 formed in the center of the support member 10,a pair of shafts or rods 12, 12 formed at both ends on the center line(chord line P--P') of the support member 10, a bolt 13 provided in themiddle of the lower surface of the support member 10, a weight 14engaged with the bolt 13 and a stopper pin 15 for preventing the weight14 from slipping out.

As shown in FIGS. 3A and 3B, substrate 1 to be processed (in FIG. 4A, asemiconductor wafer is shown as the substrate 1 to be processed) isplaced in the substrate holding part 11 in the support member 10, withits principal plane in a horizontal, upwardly facing direction. In thisposition the viscous substance 5 is applied on the surface of thesubstrate 1 as shown in FIG. 1B. Then the rods 12, 12 of the substrateholder 9 are put in the grooves 8, 8 formed in the rotor 6. Needless tosay, rotor 6 is at rest at this time. By the action of the weight 14, asshown in the upper part of FIG. 2A and in FIG. 4A, the support member 10is held in horizontal position, so that the principal plane of substrate1 is also kept horizontal and in an upwardly facing direction with rotor6 at rest. Therefore, the viscous substance applied on the surface ofthe substrate 1 cannot leak out from the surface of the substrate 1 orbe shifted to one side of the surface of the substrate 1.

Next, using a motor (not shown) or the like, rotor 6 is rotated at highspeed in the direction indicated by arrows in FIGS. 2A and 4B about itsvertical axis O. Then by the centrifugal force applied to the weight 14and the freely rotatable support of rods 12, 12 in grooves 8, 8, thesubstrate holder 9 is permitted to rotate automatically 90 degrees fromthe stationary state, as shown in the lower part of FIG. 2B and in FIG.4B, so that the support member 10 is then orthogonal to the radius ofrotor 6 which it intersects. As a result, the centrifugal force in thedirection orthogonal to the surface of the substrate 1 is also appliedto the viscous substance applied on the surface of the substrate 1. Thiscentrifugal force acts as a kind of gravity and the state in FIG. 1B ischanged to FIG. 1C and to FIG. 1D, thereby flattening the surface of thesubstrate 1.

The gravity loading on the viscous substance by the centrifugal force isproportional to the radius of rotor 6, and is proportional to the squareof angular frequency of rotor 6. That is, supposing the mass to be m,the radius of rotor 6 to be r and the angular frequency of rotor 6 to beω, the gravity force F is expressed as

    F=m19 r·ω.sup.2

Accordingly, supposing the radius r of the rotor 6 to be 50 cm and therotating frequency f of the rotor 6 to be 42 revolutions/sec (2520revolutions/min), it follows that ##EQU1## Since the gravity g inspontaneous state is

    g=980 cm/sec.sup.2,

we obtain

    rω.sup.2 /g=3.5×10.sup.3

That is, in the processing apparatus shown in FIGS. 2 to 4, by settingthe radius of the rotor 6 at 50 cm and the rotating frequency of therotor 6 at 42 revolutions/sec, a force of about 3.5×10³ times that ofgravity (g) may be applied to the viscous substance 5. As a result, fromthe state in FIG. 1B to the state in FIG. 1D, the viscous substance 5 isswiftly moved, and a desirably flat surface is formed.

Meanwhile, it is known that such rotating speed of the rotor 6 asspecified above may be achieved easily by comparing with, for example,the maximum speed of about 100,000 revolutions/min of centrifugalseparators employed generally at present.

When a semiconductor substrate is used as the substrate to be processed,it is beneficial to enclose the rotary drive mechanism portion toisolate it from the rotating space of the rotor 6 or to keep this drivein a reduced atmosphere so that the dust generated from the rotary drivemechanism of the rotor 6 may not deposit on the surface of the substrateto be processed.

In the foregoing embodiment, a semiconductor substrate is shown as thesubstrate to be processed but dielectric substrates and metallicsubstrates may be employed as well. In short, the invention may beapplied widely where the substrate surface has fine undulations, for thepurpose of filling up the concave portions thereof with a viscoussubstance.

I claim:
 1. A method for coating a substrate having concave and convexareas in a surface thereof, said method comprising the steps of:settingsaid substrate on a substrate holder which is capable of rotation abouta central axis parallel to the plane of said substrate holder, applyinga viscous coating material to both said concave and convex areas in asurface of said substrate while said substrate is supported in ahorizontal position, and spinning said substrate holder about an axisvertical to and spaced apart from the plane of said substrate holder,whereby said substrate holder is gradually rotated about said centralaxis parallel to the plane of said substrate holder from a horizontalposition to a vertical position, wherein a centrifugal force greaterthan 1G is continuously applied to said substrate in a directionorthogonal to the surface of said substrate so as to cause said viscouscoating material in said convex areas to flow into adjacent concaveareas and the surface of said substrate is coated substantially flat. 2.A method as set forth in claim 1 wherein the volume of said coatingmaterial applied to said concave and convex areas is within a range fromless than the sum of the volumes of said concave areas to more than saidsum.
 3. A method as set forth in claim 2 wherein the volume of saidcoating material exceeds the sum of the volumes of said concave areas,said method further comprising etching the entire surface of saidcoating material so as to remove said coating material from said convexareas.
 4. A method as set forth in claim 1 wherein said substrate isweighted so that said surface of said substrate tends to face inwardlytoward said vertical axis during said rotating about said vertical axis.